For a two-wheeled motor vehicle of a sports type required to have speed and flexible controllability, a vehicle body is required to have appropriate strength and rigidity so as to secure control stability in a high-speed area. A swing arm adopted for the two-wheeled motor vehicle is manufactured by forming a plurality of members through casting and welding respective components (for example, refer to Patent Document 1).
The swing arm disclosed in Patent Document 1 has a main body coupled to a pivot shaft of a vehicle body frame and a pair of arm parts extending rearward from right and left sides of the main body, and is integrally configured by forming respective members through casting and then welding the same. For both high rigidity and weight saving, the swing arm has a hollow internal structure and is provided with a rib for partitioning an internal space. Particularly, the main body has a substantially triangular shape, as seen from a side, in which a pivot pipe into which the pivot shaft is to be inserted is configured as one corner part, and is provided with the rib for partitioning a space in the main body.
An outer wall part defining an outer surface shape of the main body and the rib are connected to the pivot pipe at the same place, and an angle between the outer wall part and the rib is an acute angle. The acute angle part (tapered shape) is a cause of cracks when forming the swing arm by the casting. Therefore, as countermeasures against the cracks, a method of forming the acute angle part into a fillet shape and securing a casting thickness has been adopted.
Patent Document 1: Japanese Patent Application Publication No. 2006-69439A
However, according to the above configuration, an unnecessary thickness is required so as to secure a casting thickness, so that a weight of the swing arm increases by the unnecessary thickness.